Group III nitride crystals such as GaN crystals and AlN crystals find tremendous utility as materials for forming semiconductor devices including light-emitting devices, electronic device, and semiconductor sensors. Improving the semiconductor device characteristics mandates that the III nitride crystals be bulk, and be low in dislocation density.
Therein, the AlN crystals are generally grown by sublimation, with there being growth by natural crystalline nucleation without employing an undersubstrate, and growth using hetero-substrates, in which crystal is grown onto an SiC substrate or other nonnative substrate (substrate whose chemical makeup differs from that of the grown crystal, ditto hereinafter) serving as an undersubstrate (cf., for example, Patent Documents 1 through 3).
In growth by natural crystalline nucleation, however, not employing undersubstrates leads to not being able to control crystal-growth orientation and not being able to carry out stabilized crystal growth, which lowers reproducibility, though crystal of low dislocation density and favorable crystallinity is obtained. Furthermore, because the crystal growth is generally carried out at high temperatures (of, for example, 2300° C. or more) the distribution of heat in the crystal-growth ambient is broad and makes the grown AlN crystal susceptible to cracking, which is prohibitive of forming bulk AlN crystal.
On the other hand, in growth employing nonnative substrates, employing a nonnative substrate (such as a SiC substrate) of large-diametric-span yields bulk AlN crystal having a diametric span equal to the nonnative substrate, and facilitates control of the crystal-growth orientation, enabling stable crystal growth to be carried out. However, due to the lattice mismatch between the nonnative substrate and the AlN crystal, the dislocation density is great, which is detrimental to the crystallinity and causes stress-induced strain in the crystal.
Accordingly, with the conventional sublimation techniques, growing bulk, low-dislocation-density AlN crystal has been challenging.
In GaN crystal growth, meanwhile, it has been reported that bulk, low-dislocation-density GaN crystal through HVPE or another vapor-phase technique may be obtained by providing a mask layer with windows onto an undersubstrate such as a sapphire or SiC substrate, and then adjusting the crystal-growth conditions to form in the growth surface of the crystal pits having a plurality of facets, and carrying out the crystal growth with the pits left present in the surface (cf., for example, Patent Document 4).    Patent Document 1: Detailed Description in U.S. Pat. No. 5,858,086.    Patent Document 2: Detailed Description in U.S. Pat. No. 6,296,956.    Patent Document 3: Detailed Description in U.S. Pat. No. 6,001,748.    Patent Document 4: Japanese Unexamined Pat. App. Pub. No. 2001-102307.